Many putters have some top marking or upper putterhead geometry indicating the intended strikepoint of the putterhead and/or the sighting aiming line (perpendicular to the strikeface). Golfers using putters without a highly visible aim line must draw an imaginary line perpendicular to the strikeface and through the golf ball to the intended target. For most golfers, a putterhead aim line provides more accurate aim than an imaginary perpendicular line from the putterhead strikeface. Generally, the longer (front to rear) and more visible the aim line is, the more accurately one can aim the putter at the target.
The Rules of Golf, as promulgated by the United States Golf Association (USGA) and the Royal and Ancient Golf Club of Scotland (the “Rules”), limit putterhead length to putterhead width. The Rules also prohibit putterhead protrusions, including those in front of the putterface and rearward from the putter face or putterhead solely for purposes of aim or alignment. Hosels or necks for connecting shafts to putterheads and bent shafts protruding forward of the strikeface are allowed. They have always been popular because it is advantageous to have the shaft axis in front of the clubhead center of gravity. This produces a static balance moment about the shaft axis which increases dynamic stability when a golfer accelerates the putterhead by applying a forward force to the shaft. This dynamic stability is most effective, by resisting putterhead rotation, when the putterhead center of gravity is directly behind the shaft axis (rather than towards the toe or heal side of it).
Since under the Rules, putterhead length (fore to aft) can not exceed putterhead width, and because the Rules do not allow appendages solely for sighting or alignment, the length of most putterhead aim lines have historically been limited to putterhead length.